Hydrocarbon conversion processes utilizing crystalline zeolite catalysts have been the subject of extensive investigation during recent years as is clear from both the patent and scientific literature. Crystalline aluminosilicates have been found to be particularly effective for a wide variety of hydrocarbon conversion processes and have been described and claimed in many patents including U.S. Pat. Nos. 3,140,249; 3,140,252; 3,140,251; 3,140,253; and 3,271,418. Aside from serving as general catalysts in hydrocarbon conversion processes, it is also known that the molecular sieve properties of zeolites can be utilized to preferentially convert one molecular species from a mixture of the same with other species.
In a process of this type a zeolite molecular sieve is employed having catalytic activity within its internal pore structure and pore openings such that one component of a feed is capable of entering within the internal pore structure thereof and being converted to the substantial exclusion of another component which, because of its size, is incapable of entering within the pores of the zeolitic material. Shape selective catalytic conversion is also known in the art and is disclosed and claimed in U.S. Pat. Nos. 3,140,322; 3,379,640 and 3,395,094.
Although a wide variety of zeolitic materials and particularly crystalline aluminosilicates have been successfully employed in various catalytic conversion processes, nevertheless, these prior art processes, in general, fell into one or two main categories. In one type of conversion process a zeolite was employed which had a pore size sufficiently large to admit the vast majority of components normally found in a charge, i.e., these materials are referred to as large pore size molecular sieves and they are generally stated to have a pore size of from 6 to 13 angstroms and are represented by zeolites X, Y and L. The other type of aluminosilicate was one which had a pore size of approximately 5 angstrom units and it was utilized to preferentially act upon normal paraffins to the substantial exclusion of other molecular species. Thus, by way of considerable over-simplification until recently, there were only two types of aluminosilicates which were available for hydrocarbon processing--those which would admit only normal paraffins and those which would admit all components normally present in a hydrocarbon feed charge. See U.S. Pat. No. 3,700,585 and Canadian Pat. No. 829,282.
The cracking and/or hydrocracking of petroleum stocks is in general well known and widely practiced. It is known to use various zeolites to catalyze cracking and/or hydrocracking processes.
Of particular recent interest has been the use of a novel class of catalysts to assist in the dewaxing of gas oils, lube base stocks, kerosenes and whole crudes, including syncrudes obtained from shale, tar sands and coal hydrogenation. U.S. Pat. No. 3,700,585 discloses the use of ZSM-5 type zeolites to efficiently catalyze dewaxing of various petroleum feedstocks.
U.S. Pat. No. 3,700,585 discloses and claims the cracking and hydrocracking of paraffinic materials from various hydrocarbon feedstocks by contacting such feedstock with a ZSM-5 zeolite at about 290.degree. to 712.degree. C., 0.5 to 200 LHSV and with a hydrogen atmosphere in some cases. This patent is based upon work on the dewaxing of gas oils, particularly virgin gas oils, and crudes although its disclosure and claims are applicable to the dewaxing of any mixture of straight chain, slightly branched chain and other configuration hydrocarbons. The catalyst may have a hydrogenation/dehydrogenation component incorporated therein.
Other U.S. patents teaching dewaxing of various petroleum stocks are U.S. Pat. No. Re. 28,398; U.S. Pat. Nos. 3,852,189; 3,891,540; 3,894,933; 3,894,938; 3,894,939; 3,926,782; 3,956,102; 3,968,024; 3,980,550; 4,067,797 and 4,192,734.
Catalytic hydrodewaxing can be considered to be a relatively mild, shape selective cracking or hydrocracking process. It is shape selective because of the inherent constraints of the catalyst pore size upon the molecular configurations which are converted. It is mild because the conversions of gas oil feed to lower boiling range products is limited, e.g., usually below about 35 percent and more usually below about 25 percent. It is operative over a wide temperature range but is usually carried out at relatively low temperatures, e.g. start of run temperatures of about 270.degree. C. are usual.
An advance in hydrodewaxing was disclosed in U.S. Pat. No. 4,446,007 (Smith), which is incorporated herein by reference. Smith recognized that the dewaxing process could be a source of high octane byproduct gasoline, provided that the temperature was raised relatively rapidly to at least 360.degree. C. Rapid temperature increase after startup meant that there was some over dewaxing of the chargestock, but this was not harmful, and indeed even increased the blending value of the heavy fuel produced. More significantly, the byproduct gasoline was both high octane, and relatively low in aromatic content.
Smith observed that during operation, the mild drop in temperature associated with fresh hydrodewaxing catalyst rapidly diminished, and that hydrogen consumption could be reduced, and reactor delta Ts would approach zero, within about a month after startup.
Shape selective catalytic hydrodewaxing such as practiced in U.S. Pat. No. 4,446,007, to produce heavy fuel oil product is not usually considered endothermic or exothermic. Usually reactor temperatures at the outlet roughly equal the inlet temperature. Although the process is a catalytic hydrocracking process, some catalytic hydrodewaxing units create hydrogen rather consume it. They can create H.sub.2 because a long chain paraffin is cracked into two or more olefinic fragments. This makes H.sub.2. The olefins may or may not be saturated before they leave the hydrocracking reaction zone, and this saturation consumes hydrogen.
To summarize, shape selective catalytic hydrodewaxing to produce fuels is an unusual hydrocracking process in that there is not much temperature change through the reactor, there is not much hydrogen consumption, and it is usually conducted in a single stage. "Single stage" means that dewaxing is customarily conducted in one large reactor, or in several reactors in series, with no intermediate heating, cooling, removal of impurities, etc. between reactor beds. This is in contrast to conventional hydrocracking processes, which usually operate in several stages, with one or more quench stages to prevent temperature runaway.
We realized that catalytic dewaxing unit as proposed in U.S. Pat. No. 4,446,007 (Smith), would give an optimum startup, but not necessarily an optimum operation thereafter. The rapid startup procedure of Smith solved the problem of making the dewaxing unit an efficient generator of high octane gasoline during startup, but did not solve the problem of working the catalyst to the maximum extent possible or extending the run length. We discovered a way to make the dewaxing unit produce even more high octane gasoline, and/or last for a longer period of time, than had heretofore been thought possible.
In commercial shape-selective dewaxing units, the process is typically operated in the presence of hydrogen to minimize the amount of coke that is deposited on the catalyst. Dewaxing units typically operate with about 2000 SCFB H.sub.2, and a hydrogen partial pressure of around 300-1000 psia, with most operating at a total pressure of about 500 psig with 400 psia H.sub.2 partial pressure.
The hydrogen was believed to be of only minor importance in the wax cracking reactions. The hydrogen was thought to minimize to a great extent the amount of coke laydown that occurs on the catalyst.
Operation with more than 2000 SCFB did not significantly extend catalyst lives as compared to operation with 1600-2000 SCFB, so this is where most commercial catalytic dewaxing units operate.
At 1600 SCFB H.sub.2 addition there is an increased rate of coking, but one which many units can tolerate (with the price of a somewhat shortened catalyst life).
Operation with less than 1500 SCFB H.sub.2, e.g., 1300-1400 SCFB H.sub.2 leads to a rapid increase in coke production.
We analyzed the coke distribution in a commercial dewaxing reactor by observing coke burn during catalyst regeneration. We realized that the coke distribution pattern was highly skewed to the front end of the process. This was consistent with other refiner's experience. In the particular unit studied, the catalyst was split between two reactor vessels, 1/3 of the catalyst being in the first reactor and 2/3 of the catalyst being in the second reactor. Both reactors operated in series, and operated as a single stage.
By operating conventionally, with all of the hydrogen added being added to the first reactor, and none being added intermediate the first and second reactor, the process operated satisfactorily, but not for as long a period as desired. At the end of the first cycle, the first reactor was heavily coked, while the second reactor had less coke. Decoking the first reactor took about 100 hours, while the second reactor, which had twice as much catalyst, was decoked in about 30 hours.
The first reactor became coked, and lost activity, requiring a much higher temperature to achieve the specification product pour point. Despite the higher temperature the second reactor was never worked as hard as it could have been (based on its low coke content). We realized that the shape selective dewaxing catalyst in the second reactor had never been pushed to the maximum extent possible, i.e., at shut down there was still a very low coke level on the catalyst.
We knew that we could extend the cycle length by minimizing coke formation in the first reactor, i.e., by increasing hydrogen partial pressure or increasing hydrogen addition rates above the 1600-2000 SCFB. The drawback to such approach was that increasing the pressure increased the capital expense of the plant, while increasing the hydrogen circulation rates increased both the load on the recycle gas compressor and the duty on the fired, recycle gas heaters. This increased both the capital cost of the plant (for a large compressor and associated piping necessary to circulate the hydrogen) and increased the operating expense (recycle gas compression and heating are two of the largest utility expenses in a dewaxing plant).
We could not afford the capital and operating expenses associated with extending run length by merely increasing the unit pressure or increasing the recycle gas circulation rates. We realized an unconventional approach to hydrogen addition was necessary in order to achieve maximum utilization of the shape selective zeolite dewaxing catalyst, and to minimize the cost of H.sub.2 circulation.
Accordingly, the present invention provides in a process for the shape selective catalytic dewaxing of a wax containing feed comprising at least one of atmospheric gas oil and vacuum gas oil by passing the feed with about 1000 to 5000 SCFB of hydrogen over a dewaxing catalyst comprising a shape selective zeolite to produce a dewaxed product and wherein the hydrogen is added to retard catalyst aging, the improvement comprising conducting the dewaxing reaction in at least two stages, with a first stage containing at least 20 wt. % of the dewaxing catalyst and at least 20 wt. % of the dewaxing catalyst is in a stage downstream of the first stage, and adding at least a portion of the hydrogen downstream of the first stage reactor.